There is growing pressure for producers of greenhouse gases to dramatically reduce their atmospheric emissions. Of particular concern is the emission of carbon dioxide (CO2) into the atmosphere. One method of reducing atmospheric CO2 emissions is through its capture at a point source and subsequent storage in geological or other reservoirs.
The process for capturing CO2 from power station and other combustion device flue gases is termed post combustion capture (PCC). The most mature commercially available technology for PCC is solvent-based chemical absorption/release of CO2. When the widespread rollout of PCC technology is realized, enormous quantities of solvent such as ammonia and amine will be required. To put this in perspective, a typical 2.4 GW generator burning pulverized black coal produces approx. 30-50 tons CO2/min, or 680 kmol/min. The potential environmental impacts of solvents and solvent degradation products (produced via oxidative and thermal processes) needs consideration, especially as release to the local environment through solvent slippage at this scale may be inevitable.
Chemical absorption of CO2 may be performed with amine based processes and alkaline salt-based processes. In such processes, the absorbing medium reacts with the dissolved CO2. Amines may be primary, secondary, and tertiary. These groups differ in their reaction rate, absorption capacity, corrosion, degradation, etc. In alkaline salt-based processes, the most popular absorption solutions have been sodium and potassium carbonate. As compared to amines, alkaline salt solutions have lower reaction rates with CO2.
With primary and secondary alkanolamines the nitrogen atom reacts rapidly and directly with carbon dioxide to bring the carbon dioxide into solution according to the following reaction sequence:2RNH2+CO2⇄RNHCOO−+RNH3+  (1)
where R is an alkanol group. The carbamate reaction product (RNHCOO−) must be hydrolysed to bicarbonate (HCO3−) according to the following reaction:RNHCOO−+H2O⇄RNH2+HCO3−  (2)
In forming a carbamate, primary and secondary alkanolamine undergo a fast direct reaction with carbon dioxide which makes the rate of carbon dioxide absorption rapid. In the case of primary and secondary alkanolamines, formation of carbamate (reaction 1) is the main reaction while hydrolysis of carbamate (reaction 2) is a secondary consideration. This is due to stability of the carbamate compound, which is caused by unrestricted rotation of the aliphatic carbon atom around the aminocarbamate group.
Unlike primary and secondary alkanolamines, tertiary alkanolamines cannot react directly with carbon dioxide, because their amine reaction site is fully substituted with substituent groups. Triethanolamine ((HOCH2CH2)3N) and methyldiethanolamine ((HOCH2CH2)2NCH3) are examples of tertiary alkanolamines which have been used to absorb carbon dioxide from industrial gas mixtures. Molecular structures of sterically hindered amines are generally similar to those of non-hindered amines, except sterically hindered amines have an amino group attached to a bulky alkyl group. For example, 2-amino-2-methyl-1-propanol (NH2—C(CH3)2CH2OH). Instead, carbon dioxide is absorbed into solution by the following slow reaction with water to form bicarbonate.R3N+CO2+H2O—HCO3−+R3NH+  (3)
In order to increase the rate of CO2 absorption, especially for aqueous tertiary alkanolamine solutions, promoters have been added to the solutions. Promoters such as piperazine, N,N-diethyl hydroxylamine or aminoethylethanolamine, is are added to an absorption solution (chemical or physical solvent). Yoshida et al. (U.S. Pat. No. 6,036,931) used various aminoalkylols in combination with either piperidine, piperazine, morpholine, glycine, 2-methylaminoethanol, 2-piperidineethanol or 2-ethylaminoethanol. EP 0879631 discloses that a specific piperazine derivative for liquid absorbent is remarkably effective for the removal of CO2 from combustion gases. Peytavy et al. (U.S. Pat. No. 6,290,754) used methyldiethanolamine with an activator of the general formula H2N—CnHn—NH—CH2—CH2OH, where n represents an integer ranging from 1 to 4. U.S. Pat. No. 4,336,233 relates to a process for removing CO2 from gases by washing the gases with absorbents containing piperazine as an accelerator. Nieh (U.S. Pat. No. 4,696,803) relied on aqueous solution of N-methyldiethanolamine and N,N-diethyl hydroxylamine counter currently contacted with gases to remove CO2 or other acid gases. Kubek et al (U.S. Pat. No. 4,814,104) found that the absorption of carbon dioxide from gas mixtures with aqueous absorbent solutions of tertiary alkanolamines is improved by incorporating at least one alkyleneamine promoter in the solution.
U.S. Pat. Nos. 8,609,049, 8,273,155 and 8,192,531 attempt to utilize solvent blends to increase the rate of CO2 absorption. U.S. Pat. Nos. 8,609,049, 8,273,155 and 8,192,531 attempt to overcome these problems by using catalysts and additional promoters such as carbonic anhydrase and ammonia. Use of an enzyme in a CO2 absorber situation requires additional technologies and equipment to be added to refineries.
There exists a need to effectively remove CO2 from flue gases through use of a novel amine solvent blend that does not utilize additional catalysts and promoters.